The present invention relates to a compressor dryer system. More particularly, the present invention relates to a compressor dryer system controlled based on relative humidity.
Compressed gas systems typically provide high volumes of dry, pressurized air or other gases to operate various items or tools (while a multitude of gases can be used, this application typically refers to air as a matter of convenience). The corrosion of ferrous materials is known to be a function of the moisture content to which it is exposed. There is a significant reduction in the corrosion rate below particular moisture content for any specific set of ambient conditions; this is known as the corrosion limit. Historically, refrigerated dryers have been designed to produce a specific dew point in the downstream compressed air stream when operated at specific steady state conditions. The dew point is then correlated back to the lowest temperature exhibited by the compressed air stream within the refrigerated dryer. Some adjustment to the coldest temperature for efficiency in separating entrained liquid moisture from the air stream is usually required. The operating parameters of the dryer are then manipulated to produce the adjusted cold air temperature and therefore downstream dew point.
An illustrative dryer 100 is shown in FIG. 1. Conventional systems dry the air using heat exchangers first to cool the air and lower the dew point of the air, which causes water vapor to condense out of the air, and second to reheat the air and raise the outlet temperature of the air. This system provides a relatively dry air source. Refrigerated dryer 100 includes both an air heat exchanger circuit 110 and a refrigerant heat exchanger circuit 120. Air heat exchanger circuit 110 includes an inlet 112, an air-to-air heat exchanger 114, an air-to-refrigerant heat exchanger or evaporator 116, a water separator 120a and an air outlet 118. Refrigerant heat exchanger circuit 120 includes evaporator 116, a compressor 122, a condenser 124, a throttling device 126, and a hot gas by-pass valve 128.
In operation, dryer 100 receives a high temperature, saturated, pressurized air or gas stream at inlet 112. The air or gas stream passes through an inlet side of air-to-air heat exchanger 114. The air or gas stream cools down to an intermediate temperature during which some water vapor condenses. The condensed moisture precipitates out and collects in the separator 120a.The intermediate temperature air or gas then travels through the air side of evaporator 116 where the air or gas is further cooled to a low temperature. Again, moisture condenses out of the air or gas stream and collects in the separator 120a.The low temperature air or gas then travels through the outlet side of air-to-air heat exchanger 114. This reheats the air or gas stream to a desired temperature. The air or gas stream then exits the dryer 100 at air outlet 118. Under standard operating conditions, because the desired temperature air can hold significantly more moisture vapor than low temperature air, dryer 100 provides a source of dry, unsaturated, pressurized air or gas at air outlet 118. As explained above, the gas low temperature will be monitored and the dryer 100 manipulated to maintain the low temperature at a desired point to attempt to obtain the desired downstream dew point.
The dryer 100 can be manipulated in various ways. In refrigerant heat exchanger circuit 120, refrigerant enters the refrigerant side of evaporator 116 as a cool liquid. While passing through evaporator 116, the refrigerant heats up and is converted to a gas by the exchange of heat from the relatively hot air side to the relatively cool refrigerant side of evaporator 116. The low pressure gas travels to compressor 122 where the refrigerant is compressed into a high pressure gas. The refrigerant than passes through air or water cooled condenser 124 where the refrigerant is condensed to a cool, high pressure liquid. The cool, high pressure refrigerant passes through throttling device 126 (typically capillary tubes or the like) to reduce the pressure and boiling point of the refrigerant. The cool, low pressure, liquid refrigerant than enters the evaporator and evaporates as described above.
In the illustrated embodiment, the dryer 100 is varied by utilizing a hot gas by-pass valve 128 in the refrigerant heat exchanger circuit 120. When air heat exchanger circuit 110 and refrigerant heat exchanger circuit 120 operate at or near full capacity, and the low temperature is at the desired temperature, hot gas by-pass valve 128 has no particular function. However, when the low temperature is reduced, the hot gas by-pass valve 128 functions to reduce cooling and increase the low temperature temperature. The particulars regarding the operation of hot gas by-pass valve 128 are well known in the art. Other manipulated systems are also known. For example, some systems cycle the compressor 122 on and off when the system operates at less than 100% capacity or alternatively utilize a variable speed drive to vary the speed of the refrigerant compressor 122. Alternatively, the air heat exchanger circuit 110 may be controlled, for example, by throttling the compressed fluid flow rate through the air heat exchanger circuit 110. These systems are generally configured to operate under standard operating conditions.